The present invention relates to a coordinated brake control system of a hybrid brake system having a regenerative brake unit and a friction brake unit, and more particularly to a coordinated brake control system which is capable of smoothly transiting a braking from a regenerative braking to a friction braking.
Japanese Patent Provisional Publication No. 2000-225932 discloses a coordinated brake control system which is arranged to calculate a target barking torque according to vehicle driving and traveling conditions and to achieve the target braking-torque by a combination of a regenerative braking and a friction braking of a hybrid brake system. The hybrid brake system comprises a regenerative brake unit which generates a braking torque by converting a wheel rotating energy into electric power using a motor/generator and a friction brake unit which generates a braking torque by hydraulically or electro-magnetically operating a friction brake device.
When this coordinated brake control system generates a master cylinder hydraulic pressure Pmc at a moment t2 in FIGS. 12A through 12E in reply to the depression of a brake pedal, a regenerative braking-torque is mainly produced for a period of a relatively high vehicle speed range from the moment t1 to a moment t2 during when it is possible to accurately execute the regenerative braking so as to improve a fuel consumption by improving an energy recovery rate while suppressing a friction braking torque. Thereafter, when vehicle speed VSP becomes a relatively low vehicle speed at the moment t2 and later, the regenerative braking-torque command value is gradually decreased according to the lowering of the vehicle speed VSP as shown by a continuous line in FIG. 12C, and simultaneously a braking hydraulic pressure command value corresponding to friction braking torque is gradually increased as shown by a dot and dash line in FIG. 12D. Finally, all of the braking becomes dependent on the friction braking torque at a moment t3, and the coordinated brake control is terminated at the moment t3.
This coordinated brake control system is arranged to keep a decreasing rate for decreasing the regenerative braking torque at a constant rate. Accordingly, even if the decreasing rate is varied, there is still caused the following problem at the moment t2 and adjacent thereof when a gradual decreasing of the regenerative braking torque is started. That is, the friction braking system poorly performs a responsibility of generating the braking-torque in reply to the received signal as compared with that of the regenerative braking system, and therefore the rising of the brake hydraulic pressure at a start from zero largely delays as compared with the standard response shown by a broken line determined based on the brake hydraulic pressure command value.
This delay of the initial rising causes a large difference between the standard response and the actual response of the hydraulic pressure during an early period of the hydraulic pressure rising. Although the actual hydraulic pressure is then quickly risen by the execution of a feedback control based on the difference so as to follow up the standard response as shown by a continuous line in FIG. 12D, the difference during the early period of this transition stage shown by hatching in FIG. 12D generates a shortage of the hydraulic pressure, and therefore there is a possibility that a driver has an impression that the transition from the regenerative braking to the friction baking is not smooth.